Abstract: Climatic characteristics of different stages recorded by high-resolution magnetic susceptibility are discussed based on loess-paleosol sequence of Garzê A section and also on magnetostratigraphy on the southeastern margin of the Qinghai-Tibetan Plateau.Periodicities of the earth's orbital parameters are detected by power spectral analysis and the evolution of the southwest monsoon on orbital time-scale is studied.The results show that (1) the cycle of quasi-100 ka does exsit in the evolution of the southwest monsoon, and the obliquity cycle and precession cycle are remarkably detected as well;(2) the beat frequency periods of the precession are detected and give a strong non-linear response to the solar insolation and the tropical climatic characteristics of the southwest monsoon;(3) the mid-Pleistocene transition which was recognized in the evolution of the southwest monsoon began approximately 0.95 MaBP and ended around 0.6 MaBP.